1. Field of the Invention
The invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response maximum-likelihood (PRML) detection, and more particularly to a method and apparatus for asynchronous servo identification (SID)/address mark detection for PRML data detection.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
In direct access storage devices (DASD's), it is necessary to measure the radial and circumferential position of the transducer heads flying above the disk surfaces. Radial position information is used to locate and maintain the head over circular tracks on the disk surface. Circumferential position information is used to identify the start of different information fields around the track circumference. The accuracy and reliability of head position measurements is very important, since poor tolerance in these measurements will degrade the performance and storage capacity of the DASD.
In known DASD implementations, the detection of special matchless, previously written readback waveforms furnish unique "timing marks" used to identify circumferential position. The technique of recording timing mark information on the disk surface is widely used because of its inherent accuracy and tracking. One common type of timing mark recorded on the disk surface is the servo identification (SID). SID patterns are written only once during manufacture and can serve two purposes. The main purpose of the SID is to identify the start of the servo position error signal (PES) burst. The PES burst is used in the calculation of the radial position of the head. The other purpose of the SID can be to identify track and sector types, for example, such as, index, non-index, and guardband. Identifying track and sector types is accomplished using multiple SID patterns that differ in some degree. Another common type of timing mark is the address mark (AM). AM patterns may be written many times during the operation of the DASD. The AM pattern is used to identify the start of data blocks and is used in count-key-data (CKD) architecture to support variable length data.
For example, U.S. Pat. No. 4,631,606 discloses a system for detecting multiple types of servo patterns.
Known arrangements for asynchronous timing mark detection based on peak-detection technology utilize a slope detector circuit. The slope detector circuit is supplied from a peak-detect channel to detect signal peaks in the readback waveform. Asynchronous windowing of the slope detector output is used to detect the presence of unique patterns. Unique patterns are derived using long magnet lengths which violate run-length code constraints and thus force exclusiveness from all valid customer data patterns. U.S. Pat. No. 4,786,890 discloses an example PRML modulation code and run-length constraints.
With the advent of the PRML channel, the traditional timing mark detection scheme has become quite burdensome. The required slope detector available in a peak-detect channel is not available from the PRML channel and must be constructed for the sole purpose of SID/AM detection. Also, with the increased need to integrate electronics for smaller DASD, the inefficiencies of the traditional SID/AM detection scheme have become quite apparent.